Electric drive systems are utilized in nearly all industries for various applications. Electric drive systems convert electrical energy into other useful forms of energy, such as mechanical work as in the case of an electric motor, or thermal energy as in the case of an electric heater. To illustrate the operation of a typical electric drive system, the operation of an electric drive system incorporating an electric motor is described.
The electric drive system controls the operational parameters of the electric motor, such as the speed and torque of the electric motor. The electric drive system generally comprises a converter and a controller in addition to the electric motor. The converter operates to convert an input utility power into a controlled power signal that is supplied to the electric motor. The controller operates to control and regulate the converter so that the converter only supplies the specified controlled power signal to the electric motor.
One particular type of electrical drive system is an induction drive system. The induction drive system utilizes an induction motor and a converter having a rectifier/inverter, often referred to as an inverter, to convert the input utility power into the controlled power signal supplied to the induction motor. The induction motor generally has three stator windings that surround a rotor. The converter switches power between each of the stator windings such that the time delay or sequence of the currents flowing in the stator windings produces a magnetic field pattern of alternating north and south poles that generates a rotational force on the rotor.
The development of advanced power electronic switching devices has enabled high frequency switching, or pulse-width modulation, of the induction motor. Pulse-width modulation increases the efficiency and flexibility of the induction motor. The high frequency switching, or pulses, enables numerous commutations during each frequency period. For example, insulated gate bipolar transistors allow switching frequencies of 2 to 20 kHz and rise times of 0.1 .mu.s. The fast rise time, or dv/dt, occurs at each switching instance and often causes several problems in conventional induction drive systems.
One such problem with conventional induction drive systems is that the high dv/dt causes differential mode and common mode dv/dt current leakage losses. The higher the dv/dt, the greater the differential and common mode dv/dt current leakage losses. A high common mode current leakage can interfere with ground fault protection system in an industrial facility. In addition, a high dv/dt at the motor terminals causes stress on the motor as well as creating unwanted electromagnetic fields. The high dv/dt at the motor terminals may contribute to winding failures, insulation failures, and early bearing wear due to bearing currents induced in the rotor.
Long leads between the output of the inverter and the induction motor exacerbate the problems with conventional induction drive systems. Long leads are susceptible to a transmission line effect in which the voltages at the motor terminals are much higher than the voltages at the output of the inverter. The increase in the dv/dt at the motor terminals drastically increases the differential and common mode dv/dt leakage currents as well as the adverse effects on the induction motor.